Considerable evidence indicates that the 24 hr. fluctuations observed in behavioral, physiological and cognitive functions depend upon endogenous rhythmic mechanisms. In mammals, the generation of these 24 hr. or circadian rhythms is controlled by neural systems. Previous work indicates that the mammalian suprachiasmatic nucleus (SCN) is a very important component of the neural mechanism responsible for circadian rhythms. Therefore, a better understanding of the pathways connecting the SCN to the rest of the brain should increase our knowledge about the circadian regulation of behavior. One objective of the proposed research is to investigate the functional anatomy of the efferent connections of the SCN. To achieve this goal, selective knife cuts will be made to interrupt different hypothalamic pathways, and the effects of surgery on circadian rhythms will be evaluated. These experimental animals (rats) will be observed in constant environmental conditions. A second objective is to study the anatomical organization of the SCN in normal animals and inmutant strains of rodents. Neuroanatomical experiments, using retrograde labeling methods, will be performed to identify the neurons of origin of SCN efferent fibers. In addition, mutant strains will be studied to determine if neuroanatomical anomalies in the SCN are correlated with deficits in the expression of circadian rhythms in behavior. Research on circadian rhythms has expanded the traditional concept of homeostatic regulation and is of potential interest to medicine and psychiatry.